Effect of season and sex on the photopic spectral sensitivity of the three-spined stickleback.

It is now almost two decades since Tinbergen and his associates (Tinbergen, 1951; Tinbergen & van Iersel, 1947; van Iersel, 1953) identified the relevant features of visual displays which trigger sexual and parental behaviour in the three-spined stickleback. Specifically, they showed that during the breeding season, when the female's abdomen is swollen with eggs, the latter serves as a visual sign stimulus which triggers appropriate sexual behaviour in the sensitized male. In addition, the red colouring which the belly of the male acquires during this period was found to elicit, in other sensitized males, the complex behaviour patterns which characterize the defence of territory in this species. Presumably, these complex behaviour patterns arise during the spawning period because sexual hormones are released into the circulation and bring about the sensitization of specific circuits within the central nervous system. However, it is perhaps as well to emphasize that the present evidence does not permit a distinction to be made as to whether the action of the sexual hormones is (1) a ' facilitatory' one which operates by lowering the firing threshold of appropriate neural circuits; or (2) whether the action is inhibitory upon certain activity, raising the threshold of circuits which are inappropriate; or yet indeed (3) whether the hormones provide a specific combination of facilitation and inhibition within the centres. These questions, though of interest, are difficult to resolve experimentally. In spite of the inherent difficulties confronting all studies relating to this problem, the present authors entertained the possibility that perhaps the sexual hormones which are released into the circulation during the spawning period may conceivably also act upon the retina itself where they may initiate detectable changes that lead to a shift in overall spectral sensitivity of the retina towards the longer wavelength end of the spectrum. This would effectively lower the visual threshold for red objects and perhaps contribute to the effectiveness of red stimuli in releasing appropriate sexual behaviour. Two possibilities were considered. First, if Gasterosteus belongs to the group of fishes that possess a paired visual pigment (i.e. their visual pigment contains a mixture of retinene! and retinene2) then a shift in spectral sensitivity towards the longer wavelength end of the spectrum might rise because the action of the sexual